Variation in the Red Coloration of the Antebrachial Scales in the Male Ornate Box Turtle (Terrapene ornata ornata)

The ornate box turtle, Terrapene ornata ornata, displays significant sexual dimorphism. Males generally have thicker and longer tails, modified hind toes, a concave plastron, more muscular hind legs, and vividly red or orange irises and antebrachial scales, as opposed to the pale yellows and browns of juveniles and females (Legler, 1960). Because sexually dimorphic traits are assumed to be sexually selected, the red coloration found on males may thus indicate a male’s condition or status as he moves from pre-reproductive through the prime years for mating and on toward senescence. The purpose of this study is to quantify this “redness” quality of the antebrachial scales of male turtles, and to observe how this varies with the males’ age and size.

I analyzed photos of male turtles using the Color Transformer plug-in for ImageJ software. This plug-in converted RGB images to the CIE l*a*b* color space, where the redness of the image could be viewed and quantified. The standard deviation of each scale proved to be the best indicator of apparent redness. I estimated a turtle’s age based on the level of wear of the shell, which is in close correlation with a turtle’s known minimum age. Redness was not correlated with size, but was parabolically correlated with estimated age (R2= 0.29), suggesting that redness reaches its peak during the middle of the turtle’s life span. This correlation is weak, suggesting that other factors may be at play, such as general health and nutritional status.